1. Field of the Invention
The present invention generally relates to a head positioning system and, more particularly, to a head positioning system of a disk drive with a servo writing function.
2. Description of the Related Art
In general, a disk drive that represents a hard disk drive incorporates a head positioning system. The head positioning system positions a head in a target position on a disk medium (simply referred to as a disk hereinafter) based on servo data recorded on the disk.
As a servo writing method of recording servo data on a disk, there is a self-servo writing method that is executed using a mechanism incorporated into a disk drive (see Jpn. Pat. Appln. KOKAI Publication No. 8-212733, for example).
In the self-servo writing method, a head is positioned using servo data written to a disk and the next servo data is written thereto. The servo data contains position error components that are synchronized with the rotation of the disk, or repeatable run-out (referred to as RRO hereinafter). If the RRO increases, the distortion of old servo data in the radial direction of the disk affects that of subsequent servo data.
The prior art self-servo writing method described above has the technique of suppressing the increase of RRO by a correction function of inputting a correction value by feed forward (FF) (which may be referred to as FF correction hereinafter).
The correction function employs a linear filter (FF filter) operation that corresponds to the inverse function of the sensitivity function of a head positioning system. The sensitivity function of a feedback control system is a transfer function 1/(1+G) where G means a open-loop transfer function of the feedback control system.
In the correction function, a correction value computed by the FF filter operation is added to the input value of a controller (follow-up control element) of the head positioning system. The correction value is computed based on RRO included in position errors of the head (relative to a target position), which are sensed before the correction.
In the above correction function, however, the RRO caused after a correction value is input cannot be corrected. It is thus difficult to correct the distortion of servo data to be recorded with high precision.